FIG. 9 is a typical depiction schematically showing a structure of a generally used image forming apparatus 1 of an electrophotographic system for use in printers, copying machines, etc., which is provided with an image forming use rotatable member and a transfer drum. The image forming apparatus 1 shown in FIG. 9 is provided with a photoreceptor drum 2 (image forming use rotatable member) and a transfer drum 3. The photoreceptor drum 2 is provided for visualizing an image to be printed into a toner image by a charger (not shown), a latent image forming use optical system and a developer device, etc. The transfer drum 3 is provided for transferring the toner image to a recording material such as paper, an OHP sheet, etc., to be permanently affixed to the recording material by a fuser (not shown).
As described, the image forming apparatus 1 of the electrophotographic system includes rotatable members such as the photoreceptor drum 2 serving as the image forming use rotatable member, the transfer drum 3, etc. These drums 2 and 3 rotate as respective gears 2a and 3a formed on one end of the photoreceptor drum and one end of the transfer drum 3 interlock with each other respectively. Specifically, a drive force from a motor 5 is transmitted to at least either one of these drums 2 and 3 (the photoreceptor drum 2 in the example arrangement shown in FIG. 9) via a drive transmission mechanism 6 composed of, for example, belts, pulleys and a gear string (belts and pulleys in the example arrangement of FIG. 9), thereby rotating the rotatable members of the photoreceptor drum 2 and the transfer drum 3.
It may be arranged such that the motor 5 is driven under a control of a drive circuit 7, and the motor 5, the drive transmission mechanism 6 and the drive circuit 7 serving as rotatable member drive means simultaneously drive a rotation mechanism such as a developer, a fuser, etc., (not shown) and a recording material transportation system of a recording material, etc.
Therefore, depending on factors such as a precision in shape of the drive transmission mechanism 6 such as the gears, the pulleys, etc., variations in drive torque of the motor 5, variations in load of the rotation mechanism or the transportation mechanism, etc., variations may occur in rotation speed of the drums 2 and 3. Such variations in rotation speed may cause a warp in length of the recording material in a transportation direction, resulting in a significant degrading of an image quality. Such problems can be suppressed to some extent by selecting an optimal design, for example, for improvements of the precision and the rigidity of the drive transmission mechanism 6, and suppression of the torque deviations of the motor, etc. However, it is difficult to eliminate the described problem of variations in rotation speed.
Typical conventional arrangements for suppressing variations in speed include, for example, those disclosed by Japanese Unexamined Patent Publication No. 120582/1992 (Tokukaihei 4-120582), Japanese Unexamined Patent Publication No. 72836/1993 (Tokukaihei 5-72836), Japanese Unexamined Patent Publication No. 140841/1995 (Tokukaihei 7-140841), etc. In the above prior arts, a dynamic vibration reducer is coaxially provided with a photoreceptor drum, and variations in speed are reduced by reducing vibrations by the photoreceptor drum.
However, in the above prior arts, since the dynamic vibration reducer is integrally provided with the expendable photoreceptor drum which is needed to be exchanged periodically. This means that every time the photoreceptor drum is exchanged, the dynamic vibration reducer which can be still used has to be exchanged together which raises a problem of an increase in maintenance cost.
On the other hand, in order to provide the dynamic vibration reducer separately from the photoreceptor, a coupling member is required for mounting the dynamic vibration reducer to the photoreceptor drum. In this case, for the coupling material, if a material of a small rigidity and a small mounting strength of the dynamic vibration reducer with respect to the photoreceptor drum, desirable effects of suppressing variations in speed, i.e., suppressing vibrations may not be achieved. Moreover, additional vibrations may occur by the use of the coupling member. For this reason, for the materials of the coupling member, materials of high rigidity and mounting strength are required. For this requirement, the coupling member becomes large in size and heavy in weight, thereby increasing costs. Furthermore, it is required to take the suppression of vibrations generated from this additional member of the coupling member into consideration when designing, and the design choice of such apparatus is thus restricted.
In order to counteract the described problem, another arrangement is disclosed by Japanese Unexamined Patent Publication No 287070/1992 (Tokukaihei 4-287070) wherein a vibration reducer member is provided in a transfer drum which is not needed to be exchanged.
FIG. 10 is a perspective view of a transfer drum 11 having the described vibration reducing structure. FIG. 11 is a cross sectional view in an axial direction of the transfer drum 11. As shown in FIG. 10, the transfer drum 11 is arranged such that an elastic conductive sponge 13 is formed on a peripheral surface of a main body drum 12 made of metal, and further a conductive rubber 14 is formed on the peripheral surface of the conductive sponge 13.
As shown in FIG. 11, the main drum 12 includes a boss 16 formed at a center of a gear 15 mounted at one end of the main body drum 12, and a boss 18 formed at a center of an end plate 17 mounted at the other end. These bosses 16 and 18 are rotatably supported by the support member of the image forming apparatus main body. The gear 15 is in mesh with the gear formed at one end of the photoreceptor drum (not shown), and the photoreceptor drum and the transfer drum 11 rotates as being interlocked with each other, and variations in speed, i.e., vibrations generated from the drive transmission mechanism are reduced by the conductive sponge 13, thereby suppressing a degrading of an image quality due to the vibrations.
In the transfer drum 11 of the described prior art, however, vibrations generated from the drive transmission mechanism, the motor, etc., are reduced by the elastic deformation and viscous damping of the conductive sponge 13. Therefore, although the vibrations in various frequencies can be suppressed uniformly, peaky vibrations of natural frequency .omega.0 generated from a specific gear or an entire drive transmission mechanism shown in FIG. 12 cannot be reduced effectively.